A recently emerging class of anti-cancer therapies has focused on blocking the growth of new blood vessels or angiogenesis, which is necessary to support tumor growth. This proposal is based on a clinical clue provided by a unique population, individuals with Down syndrome, and seeks to define the molecular mechanisms underlying the cancer protection observed in the Down syndrome population. Epidemiological data suggests that a gene(s) present in 3 copies on chromosome 21 exerts a broad anti-cancer effect by controlling some common aspect of tumorigenesis. This proposal focuses on one promising candidate for this activity - the Down syndrome candidate region-1 (Dscr1) gene. The Dscr1 gene encodes a protein that blocks angiogenesis and specifically endothelial cell activation by negatively regulating VEGF-calcineurin signaling. We hypothesize that trisomic expression of Dscr1 in Down Syndrome suppresses tumor growth by attenuating VEGF-calcineurin-NFAT signaling and restricting tumor angiogenesis. In Specific Aim 1, we will determine whether over-expression of DSCR1 is responsible for suppression of tumor growth in Down syndrome by using genetically engineered mouse models recently generated in my lab. In Specific Aim 2 we will examine the role of thrombospondin-1 (TSP-1), a recently identified target in my lab of VEGF-calcineurin-NFAT signaling in endothelial cells, in modulating tumor angiogenesis. We will confirm the mechanism of Tsp-1 regulation by VEGF-calcineurin signaling and determine its role in modulating the pro-angiogenic effects of VEGF. We will also determine the consequences of Tsp-1 loss on VEGF signaling in endothelial cells. In Specific Aim 3, we will determine the differential mechanisms by which DSCR1 and the pharmacologic calcineurin inhibitor cyclosporin A, block calcineurin function using a combination of biochemical and mutational approaches and transplantable tumor models in mice. The experiments proposed will use this clinical insight to derive mechanistic insight into how tumor angiogenesis and ultimately tumor growth can be inhibited or reversed by modulating calcineurin activity and function. Finally these studies will also establish whether Dscr1 is a valid target for anti-cancer intervention.